I haven't changed anything in my setup hardware or software wise, usually got around 20-30 connections immediately for the past 6 weeks. But now suddenly I'm stuck at 0 connections, sometimes it will go up to 1 or 2 but then quickly drop back to 0 without downloading the latest block. I'm stuck at 136916 here.

http://canyouseeme.org/ check for port 8333. if you get the green light then id just reformat, but if you don't, try port forwarding or temporarily hooking your modem into your computer to see if it starts updating.

I did this, and also added another 4 fallback nodes from the wiki. After restart I had 1 connection for a second, then it jumped back to zero and stayed there. Some minutes later I again had 1 connection for 10 seconds till it jumped back to zero. Is something actively killing my conns?

I really never had any problems with connections, but now that I make a huge transaction suddenly everything goes wrong? Please help me :/

--- ctoon6: I get this when checking:

Error: I could not see your service on 91.65.xxx.xxx on port (8333)Reason: Connection timed out

I'm using the client built-in upnp for port forwarding and never had any problems getting lots of connections (over 6) before. Also I'm on cable internet so I can't really go around the router.

And another strange thing happened: I'm now stable at exactly 1 connection for the last 15 minutes and the client even managed to download the latest blocks. Shouldn't it now start to acquire more connections, since it's obviously not really blocked and connected to the network successfully? This all doesn't make any sense to me.

Yes, I did the test. Port 8333 is still not open to the outside (which is correct imo, since it's only opened for the bitcoin client via upnp, right?), I am still connected to one lonely node, blocks are downloaded successfully, but no more than this one connection get opened.